Optical fiber non-reciprocal phase shifters and optical isolators using them

Abstract

In accordance with the invention, an optical fiber phase shifter comprises a length of asymmetrical optical fiber disposed perpendicular to a magnetic field for providing a non-reciprocal phase shift. Preferably the fiber is wound in a circular coil and exposed to an axial field as from a pancake magnet. In an isolator, the fiber length and magnet strength are chosen to provide non-reciprocal phase shifts of ±45° or ±90°. The isolator including the phase shifter can be any of several configurations.

We claim: 
     
       1. An optical fiber non-reciprocal phase shifting device comprising: a length of optical fiber having a core and a cladding and a longitudinal direction for light propagation;   a magnet for applying a magnetic field to said optical fiber in a direction transverse to said longitudinal direction; said longitudinal direction and the direction of said magnetic field defining a plane; and   said core being asymmetric with respect to said plane for producing a non-reciprocal phase shift in light propagating through said fiber.   
     
     
       2. A device according to claim 1 wherein said length of optical fiber is in the form of a coil and said magnetic field is in the direction of the axis of said coil. 
     
     
       3. A device according to claim 1 or 2 wherein the cladding of said optical fiber is provided with one or more planar surfaces. 
     
     
       4. A device according to claim 2 wherein said cladding is provided with at least one flattened surface and said coil is wound with said flattened surface parallel to the direction of said magnetic field. 
     
     
       5. An optical fiber Sagnac effect gyroscope comprising an optical fiber non-reciprocal phase shifting device according to claim 1 or claim 2. 
     
     
       6. A device according to claim 1 or 2 wherein said core is asymmetric due to the presence of two different materials forming two different parts of the core. 
     
     
       7. A device according to claim 1 or 2 wherein said core is asymmetric due to the presence of two different materials forming two different parts of the core separated by a sufficiently small thickness of cladding that the two parts are optically coupled. 
     
     
       8. An optical isolator comprising an optical fiber non-reciprocal phase shifting device according to claim 1 or claim 2. 
     
     
       9. An optical fiber circulator comprising an optical fiber non-reciprocal phase shifting device according to claim 1 or claim 2. 
     
     
       10. An optical fiber comprising a cladding and a core and a longitudinal direction for light propagation; said cladding having at least one flattened surface; and   said core having no plane parallel to said flattened surface which is a plane of symmetry, wherein said core is asymmetric due to the presence of two different material forming two different parts of the core.   
     
     
       11. An optical fiber according to claim 10 wherein said two different parts of the core are separated by a sufficiently small thickness of cladding that the two parts are optically coupled together.